The present invention relates to a process for producing, in a high yield and with high purity, 2,6-naphthalenedicarboxylic acid of high bulk density which is used for the preparation of polyesters such as polyethylene naphthalate and polyamides useful as a starting material for the production of films and synthetic fibers.
As methods for producing 2,6-naphthalenedicarboxylic acid (hereinafter referred to as "2,6-NDCA"), processes which 2,6-dimethylnaphthalene, 2,6-diethylnaphthalene or 2,6-diisopropylnaphthalene (hereinafter referred to as 2,6-DIPN) is oxidized with molecular oxygen in an acetic acid as a solvent, in the presence of a catalyst composed of cobalt and/or manganese and bromine are already known. (Refer to Japanese Patent Publication Nos. 43,893 (1973), 21,027 (1981), 2,223 (1983), 13,495 (1984), and 27,318 (1973); and Japanese Patent Application Laid-Open (KOKAI) Nos. 34,153 (1973), 42,654 (1974), 17,453 (1977), 89,445 (1985) and 89,446 (1985). Especially noticeable are the process in which 2,6-DIPN or its partially oxidized intermediate is oxidized with molecular oxygen in a solvent containing at least 50% by weight of an aliphatic monocarboxylic acid having not more than three carbon atoms in the presence of a catalyst composed of a heavy metal, comprising cobalt and/or manganese, and bromine, said heavy metal being used in an amount of at least 0.2 mole to one mole of 2,6-DIPN or its partially oxidized intermediate [Japanese Patent Application Laid-Open (KOKAI) No. 89,445 (1985)] and the process in which 2,6-DIPN or its partially oxidized intermediate is oxidized with molecular oxygen in a solvent containing at least 50% by weight of an aliphatic monocarboxylic acid having not more than three carbon atoms in the presence of a catalyst composed of a heavy metal, comprising cobalt and/or manganese, and bromine, said heavy metal being used in an amount of at least 1% by weight based on the solvent [Japanese Patent Application Laid-Open (KOKAI) No. 89,446 (1985)].
However, as a result of the studies on side reaction in the production of 2,6-NDCA by oxidizing 2,6-DIPN, the present inventors found out that the main side reactions are the following two reactions: (1) a cleaving reaction of the naphthalene nucleus to form trimellitic acid and (2) a polycondensation reaction. These side reactions can be suppressed to a certain extent by using a large amount of a heavy metal catalyst, comprising cobalt and/or manganese, but use of expensive cobalt or manganese in a large amount is undesirable. Also, the crystals of 2,6-NDCA obtained by using a large amount of the heavy metal catalyst are low in bulk density and is difficult to separate by filtration and, further, when dried, they are severely dusty and their transportation by belt conveyors, etc. is difficult.
The catalysts usable in the reaction for producing 2,6-NDCA by oxidizing 2,6-DIPN can largely be classified in two groups, i.e., cations and anions. As to cations, the effect of heavy metal ions such as cobalt and manganese ions and alkali metal ions such as sodium and potassium ions have been discussed [Japanese Patent Application Laid-Open (KOKAI) Nos. 246,143 (1986) and 246,144 (1986)], but as to anions, only the catalytic effect of bromine is known and no substantial studies have been made on the effect of anions.
The object of this invention is to provide a process manufacturing 2,6-NDCA having high bulk density, easy to treat and in a high yield by suppressing formation of by-product, that is, polycondensate which is common in a conventional processes.